This application is 371 of PCT/EP98/00479, filed Jan. 29, 1998.
The present invention relates to a membrane-electrode unit for a polymer electrolyte membrane fuel cell having a polymer electrolyte membrane, an anode arranged on one surface of the membrane and a cathode arranged on the other surface of the membrane, as well as to a method of making the membrane-electrode unit.
Polymer electrolyte membrane fuel cells, as they are commonly employed for producing electric current, contain an anode, a cathode and an ion exchange membrane disposed therebetween. A plurality of fuel cells constitutes a fuel cell stack, with the individual fuel cells being separated from each other by bipolar plates acting as current collectors. The bipolar plate on the anode side of a cell constitutes at the same time the cathode side bipolar plate of the neighboring cell. For generating electricity, a burnable gas, e.g. hydrogen, is introduced into the anode region, and an oxidizing agent, e.g. air or oxygen, is introduced into the cathode region. Both the anode and cathode in the regions in contact with the polymer electrolyte membrane contain a catalyst layer. In the anode catalyst layer, the fuel is oxidized thereby forming cations and free electrons, and in the cathode catalyst layer, the oxidizing agent is reduced by taking up electrons. As an alternative, the two catalyst layers may also be applied on opposite sides of the membrane. The structure of an anode, a membrane, a cathode and the corresponding catalyst layers is referred to as membrane-electrode unit. The cations formed on the anode side migrate through the ion exchange membrane to the cathode and react with the reduced oxidizing agent, thereby forming water when hydrogen is used as burnable gas and oxygen is used as oxidizing agent. The heat created in the reaction of burnable gas and oxidizing agent is dissipated by exactly fitting manner for each individual membrane-electrode unit. Inexpensive manufacture of membrane-electrode units in the form of square-meter material is not possible. Moverover, the seals must be cut separately and then attached in an exactly fitting manner. cooling. For better distribution of the reaction gases and, possibly, for supporting the membrane-electrode unit, gas-conducting structures, e.g. grid-like nets, may be provided between electrodes and bipolar plates.
Upon installation in a fuel cell, the membrane-electrode unit is in contact on the anode side with the burnable gas and on the cathode side with the oxidizing agent. The polymer electrolyte membrane separates the regions containing the burnable gas and the oxidizing agent, respectively, from each other. For preventing contact of the burnable gas and oxidizing agent, which could cause explosion-like reactions, reliable sealing of the gas spaces from each other must be ensured. In this respect, a problem is present in particular for providing a sealing against burnable gas hydrogen that has excellent diffusion properties.
In order to prevent that a gas exchange can take place in the fuel cell along the edges of the membrane, the following measures are taken conventionally: in producing conventional membrane-electrode units, the dimensions for the membrane and electrodes are selected such that, with a sandwich-like arrangement of the membrane between the electrodes, the membrane projects on each side a good distance beyond the area of the electrodes. The conventional membrane-electrode unit thus comprises a membrane with the edge portions that are not covered by electrode material. Flat seals, e.g. of stretched PTFE, are attached around the periphery of the membrane-electrode unit on both sides of the membrane so as to cover the projecting portions of the membrane. In case of a square membrane-electrode unit, for example, square frames are pressed on and/or attached adhesively on both sides of the membrane, such that they at least partly cover the projecting portions of the membrane. These conventional membrane-electrode units on the one hand involve the disadvantage that they are quite complex in manufacture since the anode, cathode and membrane must each be cut separately and then must be assembled in an exactly fitting manner for each individual membrane-electrode unit. Inexpensive manufacture of membrane-electrode units in the form of square-meter material is not possible. Moreover, the seals must be cut separately and then attached in an exactly fitting manner.
A further disadvantage of the conventional membrane-electrode units becomes evident in mounting the same in a fuel cell. In the fuel cell, a gastight space must be provided at least on the anode side between membrane-electrode unit and the bipolar plate confining the cell. Conventionally, sealing rings or strips are employed here between membrane-electrode unit and bipolar plate, with several cells each being clamped together in series and being provided with a joint supply of burnable gas. The gastight spaces are formed upon such clamping together only. In case of a leak, it is difficult to locate the same, and it is not possible either to remove just one cell, but only the clamped together unit containing the leak. This involves considerable expenditure in work and loss of useful time of the fuel cell.
Occasionally, it is dispensed with providing the membrane-electrode unit with a pressed-on sealing frame. Sealing then is effected upon installation in a fuel cell by clamping a sealing ring between the membrane part not covered by electrode material and the adjacent bipolar plate. In both cases, a gap results between the electrode material and the seal, making the arrangement sensitive to mechanical damage, in particular in case of thin or brittle membranes. Furthermore, there is the risk that the membrane-electrode unit is not clamped in a completely planar manner so that the membrane contacts the metallic current lead-out conductor. The metal then may be partly removed by an acid membrane. The metal ions enter the membrane, thereby impairing the conductivity thereof.
The present invention allows to overcome the above-indicated disadvantages.
It is the object of the invention to make available a membrane-electrode unit for a polymer electrolyte membrane fuel cell, which on at least one side can be connected to a bipolar plate in such a manner that a gastight space is formed between membrane and bipolar plate.
Another object of the invention is to make available a membrane-electrode unit in which the assembly membrane-electrode unit/bipolar plate can be tested for gas tightness separately.
A further object of the invention consists in making available a simple, inexpensive method of making such membrane-electrode units.
In making the membrane-electrode unit, according to the invention, the anode, cathode and membrane are not cut separately and the individual parts then connected to each other, but rather a layer material is produced consisting of an anode material, a cathode material and a membrane material disposed therebetween, for example by means of a rolling method as employed in paper production. This provides square-meter material from which the individual membrane-electrode units can be cut, punched or severed in another manner in one operation in the desired size. A membrane-electrode unit obtained in this manner contains, apart from the end face, no free membrane area, but rather the membrane on both surfaces thereof is fully covered by the anode material and the cathode material, respectively. If desired, passages can be formed in the membrane-electrode unit, which is possible in one operation as well.
The membranes, electrodes and catalysts used for manufacturing the membrane-electrode unit according to the invention as such may be conventional materials, as they are commonly used for corresponding purposes. As electrodes, i.e. anodes and cathodes, there may be used, for example, diffusion electrodes of carbon paper or graphitized fabrics, containing a catalyst having an arbitrary distribution parallel and also vertical to the electrode area. Instead of carbon paper or graphitized fabrics, however, nonwoven fabrics of carbon fiber material may be used as well. As catalyst, it is possible to use e.g. platinum on carbon. The electrodes may contain only the catalyst layer, part of the diffusion layer or the entire diffusion layer. As an alternative thereto, the catalyst may also be applied to the surfaces of the membrane. As membranes, the usual ion-conducting polymers, for example nafion or a sulfonized polyether ether ketone ketone (PEEKK, available from Hoechst), are employed in advantageous manner.
The membrane-electrode units in the form of square-meter material can be manufactured under procedural conditions as in case of the conventional, individual membrane-electrode units. In case of the invention, one web of electrode material each is disposed on each surface of a web of a polymer electrolyte membrane present in its H30  form, and thereafter is rolled on preferably at pressures of about 30 bar to 500 bar and temperatures of up to 250xc2x0 C. Typical pressures are between about 80 and 250 bar and temperatures between about 80 and 120xc2x0 C. When the electrode material contains the catalytically active layer, it must be rolled onto the membrane such that the catalytically active layer is in contact with the membrane.
As an alternative, it is also possible to apply one electrode first and to apply the second electrode in a second operation.
From this membrane-electrode layer material, membrane-electrode units are cut in the desired size in one operation, and at least around the periphery of each membrane-electrode unit there is formed a sealing edge that connects the membrane and the electrode or electrodes to each other in a gastight manner and, furthermore, may be connected in a gastight manner to a bipolar plate. The term xe2x80x9cmembrane-electrode unitxe2x80x9d as used herein thus, in the sense of the invention, refers to layer material pieces of anode, cathode and membrane material without or with sealing edge, in which, apart from the end face, there is in essence no membrane area present that is not covered by electrode material. The sealing edge or wear ring is provided by having a sealing agent, for example a plastic material or a mixture of plastic materials, penetrate into edge portions of the electrodes at the periphery of the membrane-electrode unit, such that the pores of the electrodes are substantially filled and no longer allow gas to pass. The plastic material, preferably a thermoplastic material or a curable liquid plastics material of low viscosity, can penetrate into the electrodes by capillary action an can then be cured, or a plastic material in liquid form, i.e. molten, uncured or dissolved in a solvent, can be pressed onto the electrode, possibly using the required pressure (preferably up to 200 bar) and/or elevated temperature in a suitable device, so as to fill the pores of the electrode in this manner. If necessary, an evacuation can be carried out before in order to remove air from the pores and thus facilitate penetration of the sealing agent. Preferred plastic materials are polyethylenes, polypropylenes and polyamides as well as epoxy resins, silicones and polyester resins. To provide enhanced wetting of the edge portions of the electrodes through which the plastic material should penetrate, it is possible, prior to the treatment thereof with plastic material, to wet the same with a solvent for the plastic material or to slightly mill the same at the surface. Partial oxidizing of the respective regions of the carbon materials of the electrodes may be advantageous as well.
The sealing edge thus formed around the periphery of the membrane-electrode unit prevents the discharge of reaction gases from the electrodes xe2x80x9cradiallyxe2x80x9d outwardly by xe2x80x9ccloggingxe2x80x9d the gas paths in the edge portion of the electrodes and, furthermore, provides for gastight glueing together and adhesion of the edge portion of the electrodes to the adjoining membrane portion.
Such seals may be provided in all regions of the membrane-electrode unit in which edges are present, e.g. also at passages through the active areas of the membrane-electrode unit, as they are often required for the supply of gases, water or for clamping screws.
As an alternative, a sealing edge can be provided by attaching a sealing frame around the periphery of the membrane-electrode units, at the substantially flush end faces thereof, or by lining passages completely or in part with a sealing frame, respectively. Possible materials for the sealing frame are preferably plastic materials or plastic material mictures, in particular thermoplastic materials such as polyethylenes, polypropylenes and polyamides or curable plastic materials such as epoxy resins, silicones or polyester resins. The sealing frame is attached in such a manner that it firmly connects to and adheres to the end faces of the membrane or the end faces of at least one electrode and the membrane, respectively, in a gastight manner.
The required passages may also be provided directly in the sealing edge itself.
Mixed forms of the two sealing edge alternatives, i.e. sealing edges formed partly within the electrode material and partly at the end faces, are possible as well.
The membrane-electrode units sealed at the periphery and possibly at passages can then be connected to bipolar plates. As a rule, bipolar plates must not rest on the electrodes directly, but there must be a free space left between electrode and bipolar plate, in which a gas-conduting structure, e.g. a net, can be provided for enhanced distribution of reaction gases across the surface of the electrode. This free space can be provided by having the sealing edge not terminate flush with the electrode surface, but designing it so as to project beyond the electrode surface, In case of electrodes with projecting sealing edge, a planar bipolar plate can be connected to the sealing edge, e.g. by gastight adhesion, or may be pressed on in the fuel cell stack under permanent pressure and thus be rendered gastight. When the sealing edge ends flush with the electrode surface, a bipolar plate may be used which is formed thinner in the portion in which the gas-conducting structure is to be applied than in the portion connected in gastight manner to the sealing edge of the membrane-electrode unit. Bipolar plates containing an integrated gas-conducting structure, such as e.g. graphite plates with milled grooves, do not require projecting sealing edges, either.
The sealing structure of the membrane-electrode unit according to the invention can be applied in general for all reaction gases. However, in particularly advantageous manner it can be employed on the hydrogen side of a fuel cell since hydrogen on the one hand has very good diffusion properties and thus presents the greatest problems in sealing, and since hydrogen on the other hand is very reactive, thereby presenting considerable difficulties in case of a leak.
The formation of a sealing edge by introduction of a sealing agent into the electrode edge portions as well as sealing of passages by introduction of a sealing agent into the electrode portions surrounding the passages is possible on principle with any membrane-electrode electrode unit with porous electrodes, irrespective of whether a flush end face termination is present. In particular, sealing according to the invention is also possible with passages, irrespective of the type of seal chosen for the outer periphery of the membrane-electrode unit.